The invention relates to a process for deforming a piece of thin-walled metal tube, comprising the steps of bending the piece of tube with respect to its original longitudinal axis and then hydroforming the piece of tube at least at the location where it has been bent in this way.
A known method for deforming pieces of tube comprises what is known as hydroforming. In this process, the wall of the piece of tube is pressed against a mould piece under the influence of water pressure, so that the piece of tube acquires its ultimate shape. The hydroforming technique is generally known and therefore does not require any further explanation here. If the piece of tube is also to be bent, a bending operation is carried out prior to and separately from the hydroforming step, in which case the bent piece of tube then acquires its ultimate, desired shape through hydroforming. In this way, numerous very complicated shapes can be produced, which are used in engineering, for example in the automotive industry.
It has been found that, in this processing method, the bending of the piece of tube forms a critical step. Particularly if a small wall thickness is used for the piece of tube, cracks are rapidly formed along the outside of the bend during the bending operation, while wrinkles are formed in the compressed region along the inside of the bend. These wrinkles in this case occur in the circumferential direction of the piece of tube, i.e. they run in a direction which is transverse with respect to the longitudinal axis of the piece of tube. If the bent piece of tube is then subjected to a hydroforming operation, it is found that wrinkles of this nature running in the circumferential direction can no longer be removed. The result is an unacceptable product.
JP-61-086029-A discloses a process for deforming a metal tube of the above mentioned kind which involves a pressure-crushing step prior to the bending of the tube. In this pressure-crushing step, the tube is fitted into a hole of a crushing device, which has the same diameter as the outer diameter of the tube, and then a punch is driven against the tube wall at the location, where the tube is to be bent in the later bending step. Thereby the section of the tube, which comes into contact with the punch, is pressure-crushed and brought into close contact with the pipe wall on the opposite side. After this pressure-crushing step the tube is inserted into a hole of a tube bending device such that the straight tube wall part, to which the pressure-crushed section of the tube wall of the tube has been brought into contact, is positioned so as to abut a crest-form apex part of the lower edge surface of the hole. Then a punch is driven through a hole of the die against the pressure-crushed section of the tube wall whereby the tube is bent along the crest-form apex part.
In this prior art process generation of creases is said to be avoided in the bending step, since the wall sections of the tube abutting the crest-form apex part are pinched-pressured from both sides during the bending process. However, in the pressure-crushing step the wall section has to be deformed to a high extent in order to bring it into close contact with the wall section on the opposite side. Accordingly this process is hardly applicable to thin-walled tubes.
Therefore, the object of the invention is to provide a method in which the production of bent, thin-walled pieces of tube and of products formed using such pieces of tube causes fewer problems. In particular, it is intended to reduce the risk of cracking and to avoid the formation of wrinkles in the circumferential direction.
In the process described in the preamble, this is achieved if the piece of tube, before it is bent, is indented on both the inside and the outside of the bend which is to be formed at the location where it will exhibit a bend as a result of the bending operation.
As a result of the indentation, which involves moving wall material closer to the neutral plane of bending stresses, the piece of tube can be bent more easily, with the result that the risk of cracking and the formation of wrinkles in the circumferential direction is reduced considerably. The ultimate shape of the piece of tube which is to be deformed can then be imposed during the subsequent hydroforming operation, during which any deformations which may have been imposed in the longitudinal direction can be eliminated, or further deformations can be produced. It should be noted that, surprisingly, it has been found that wrinkles in the wall of the bent piece of tube which run in the longitudinal direction, i.e. more or less parallel to the axis of the piece of tube, are no longer visible after the hydroforming has taken place.
The process can be used in order to impart a constant cross section to the deformed piece of tube along its length, in that the undeformed piece of tube is firstly provided with longitudinal wrinkles on either side of the neutral plane as a result of compression, and then, after the bending operation, the constant cross section is restored by the hydroforming step. It has been found that in this way pieces of thin-walled tube with a constant cross section and a relatively small radius of curvature can be produced successfully. In the past, the production of pieces of tube of this nature caused considerable problems.